I agree reinvented. The body is able to fast at any time. In nature fasts are conducted in times of drought, injury and illness, though I assume wild animals don't eat as much junk food as us so don't get the sniffles as often. None of these reasons for fasting are predetermined, they come upon us suddenly and the animal responds by avoiding food until they are healed or in the case of drought or dearth until food/water is available again.
In our case I do think that our mental preparedness, our motivation is more compelling a force in when we begin a fast than any physical need in most cases.